.alpha.-olefins are often polymerized using a Ziegler-Natta catalyst system made up of a so-called procatalyst and a cocatalyst. Of these, the procatalyst component is based on a compound of a transition metal belonging to any of Groups IVA-VIII in the Periodic Table of the Elements, and the cocatalyst component is based on an organometallic compound of a metal belonging to any of Groups IA-IIIA in the Periodic Table of the Elements (the groups are defined according to Hubbard, i.e. IUPAC). The catalyst system may also include a carrier on which the transition-metal compound is deposited and an internal electron donor which enhances and modifies the catalytic properties and is deposited on the carrier together with the transition-metal compound. In addition, a separate so-called external electron donor can also be used together with the procatalyst and the cocatalyst.
In the preparation of stereoselective high-yield Ziegler-Natta catalysts on a magnesium dihalide, organic esters are widely used as internal donors to improve the said properties of the catalyst. Publications U.S. Pat. No. 4,522,930, EP-45 977 and FI-70 028 describe an .alpha.-olefin polymerization catalyst obtained by causing a cocatalyst, an external donor and a carrier-based procatalyst to react together. The procatalyst is made up of a titanium compound deposited on a magnesium dihalide carrier and of an internal electron donor. The internal donor may be any of the following (summary of the above state of the art):
1) saturated dicarboxylic acid ester, PA1 2) unsaturated polycarboxylic acid ester, PA1 3) aromatic di- or polycarboxylic acid ester, PA1 4) aromatic oligohydroxy compound ester, PA1 5) monocarboxylic acid ester, PA1 6) araliphatic polycarboxylic acid ester, PA1 7) heterocyclic aromatic polycarboxylic acid ester, PA1 8) carbonic acid ester, PA1 9) polyol or monohydroxyphenol ester, PA1 10) acetylenic acid ester. PA1 DEMB=diethyl cis-2-methyl-butenedioic acid ester, PA1 DIBMB=di-isobutyl cis-2-methyl-butenedioic acid ester, PA1 DDMB=didecyl cis-2-methyl-butenedioic acid ester. PA1 DEMB=diethyl cis-2-methyl-butenedioic acid ester, PA1 DIBMB=di-isobutyl cis-2-methyl-butenedioic acid ester, PA1 DDMB=didecyl cis-2-methyl-butenedioic acid ester, PA1 DEME=diethyl maleate, PA1 DIBME=di-isobutyl maleate, PA1 DDME=didecyl maleate.
Regarding group 2) it is stated that the two carboxyl groups may, or may not, be linked to contiguous carbon atoms which form double bonds. As an example of internal electron donors of the former type, publications FI-70 028 and EP-45 977 mention the compounds di-2-ethyl-hexyl maleate, di-isobutyl maleate, di-isobutyl-3,4-furan-dicarboxylate, di-2-ethyl-hexyl fumarate, 2-ethyl-hexyl fumarate, and 2-ethyl-hexyl monomaleate. In addition, the FI publication states that the esters of maleic, pivalic, carbonic, and phthalic acids are especially advantageous internal donors.
According to Table I of the FI and EP publications, the activity of maleates is, however, only about one-half of the activity of phthalates, and thus there is very little use for them in commercial processes. By the same criteria it can be concluded that the di-butyl-itaconate presented in the table of the US publication is not a practicable internal donor, either.